Chemical kinetics is the study of the speed at which a chemical reaction occurs. The reaction rate measures how quickly the concentration of a reactant decreases or a product increases over time. Simply expressed, the rate is the change in concentration divided by the change in time. Measuring the rate is fundamental because it provides insight into the reaction’s mechanism, the specific sequence of molecular steps it follows. This data also allows for the optimization of industrial processes and is the basis for calculating a substance’s half-life.
Fundamental Principles of Tracking Chemical Change
Measuring a reaction’s rate fundamentally requires tracking a variable that changes predictably as the chemical transformation proceeds. This variable must be easily measurable and directly related to the amount of a substance present. The two primary strategies are monitoring the disappearance of a starting material or the appearance of a newly formed product. Since the total quantity of atoms remains constant, tracking either side of the chemical equation provides the necessary data.
A successful measurement technique relies on a distinct difference between the monitored substance and the rest of the mixture. For instance, if a reactant is colored and the products are colorless, the gradual fading of the color can be tracked. If a gaseous product is formed from liquid reactants, the volume of gas produced over time offers a clear, measurable metric. This requirement for uniqueness guides the selection of the most appropriate method for any given reaction system.
Measurement Methods Using Direct Concentration Analysis
One established method for measuring reaction rates involves determining the concentration of a reactant or product at specific, discrete time intervals. This approach is particularly useful for reactions that are relatively slow, allowing sufficient time to collect and analyze samples. The initial step is called quenching, which involves rapidly stopping the reaction after a measured time period.
Quenching is accomplished by dramatically altering the reaction conditions. This is typically achieved by rapidly cooling the sample in an ice bath, which drastically slows down molecular movement. Alternatively, a chemical inhibitor may be added to neutralize a catalyst or consume a reactant, halting the main reaction.
Once quenched, the sample’s concentration is determined using an analytical technique, most commonly titration. For example, if an acid is consumed in the reaction, the remaining acid concentration can be found by titrating the quenched sample with a standard base solution. This method yields a series of data points—concentration versus time—that are then plotted to visualize the reaction’s progress. Because this technique requires sampling, quenching, and subsequent analysis, it is more laborious and less continuous than other methods.
Continuous Monitoring Methods Using Physical Properties
Modern laboratories often favor techniques that continuously monitor a physical property without needing to stop or disturb the reaction. These methods provide a data set throughout the entire course of the reaction.
One widely used technique is spectrophotometry, which tracks changes in light absorption. If a reactant or product absorbs light at a specific wavelength, a spectrophotometer can measure the change in the solution’s absorbance over time. The absorbance is directly proportional to the concentration of the light-absorbing species, allowing for continuous and precise concentration tracking.
Reactions that produce a gas can be conveniently monitored by tracking the change in volume or pressure. If the reaction is run in a closed container, a pressure sensor can record the increase in pressure as the gaseous product accumulates. Alternatively, in an open system, a calibrated gas syringe or an inverted measuring cylinder over water can be used to measure the volume of gas released as a function of time.
Other reactions involve changes in the electrical properties of the solution. For example, if a reaction consumes ions or produces new ions, the solution’s electrical conductivity will change. A conductivity probe can continuously monitor this change, providing an indirect measurement of the concentration change. Similarly, in reactions where hydrogen ions are consumed or produced, a pH meter can track the changing acidity or basicity, offering another real-time indicator of the reaction’s progress.
Determining the Instantaneous and Average Rate
Regardless of the data collection method, the information obtained is plotted on a graph with concentration on the vertical axis and time on the horizontal axis. This results in a curve that illustrates the reaction’s progress, and its mathematical analysis yields the reaction rate.
The average rate is calculated over a large time interval by finding the slope of the secant line connecting two points on the curve. This value represents the overall change in concentration during that specific time frame. While easy to calculate, the average rate does not accurately describe the speed of the reaction at any single moment.
To find the reaction rate at a specific moment, the instantaneous rate is calculated. This is determined by finding the slope of the line tangent to the curve at that point. Since the rate of most reactions slows down as reactants are consumed, the slope of the curve gradually decreases over time.
Of particular interest is the initial rate, which is the instantaneous rate at the moment the reactants are first mixed (time zero). This initial rate is frequently used in chemical kinetics experiments because the reactant concentrations are known exactly at the start, simplifying the analysis of how concentration affects reaction speed.